Preheat oven to 350°F. Press graham-cracker crust evenly into bottom and up sides of a 9-inch pie plate. Refrigerate until firm, about 30 minutes. Bake until crust is set and begins to turn golden brown. about 12 minutes. Transfer pie plate to a wire rack and let cool completely.

Make the filling: Sift together sugar, cocoa, cornstarch, and salt into a medium saucepan. Gradually whisk in milk. Cook over medium-high heat, stirring constantly, until almost boiling. Reduce heat to medium; add chocolate. Cook, stirring constantly. until chocolate has melted and mixture is thick, about 2 minutes. Remove from heat; whisk in butter and vanilla until smooth. Spread chocolate mixture over crust. Refrigerate until cold and firm, about 1 hour.

Make the topping: Prepare an ice-water bath. In a small bowl, sprinkle gelatin over 2 tablespoons cold water. Let soften, about 5 minutes. In a medium saucepan, whisk together ¼ cup sugar, the cornstarch, and salt. Gradually whisk in milk. Cook over medium-high heat. stirring constantly, until mixture is thick and boiling, about 5 minutes. Remove from heat; stir in gelatin mixture and let cool completely. Stir in vanilla and rum. Briefly place pan in the ice bath to thicken slightly; do not let it set completely.

With an electric mixer on medium-low speed, whisk egg whites until soft peaks form. Reduce speed to low; continue beating while you make the syrup.

In a small saucepan, bring remaining ½ cup sugar and 3 tablespoons water to a boil, stirring to dissolve sugar. Brush down sides of pan with a wet pastry brush to prevent crystals from forming. Cook, without stirring, until syrup registers 240°F on a candy thermometer.

Raise mixer speed to high and beat egg whites just until stiff peaks begin to form. Immediately pour syrup down side of bowl in a slow, steady stream. Beat until meringue is glossy and cooled, about 7 minutes.

Fold meringue into gelatin mixture in three batches until just combined. Spread mixture in chocolate-filled crust. Refrigerate at least 3 hours or up to overnight (covered), before serving.